1. Field of the Invention
The present invention relates to an airbag Module, and more particularly, to an airbag device which can adjust the expansion size of a cushion according to the expansion pressure of an inflator at the time of expansion.
2. Description of the Conventional Art
Generally, an air bag for vehicles may be classified into a driver air bag (DAB) which is mounted to a steering wheel to protect the driver in a driver's seat and a passenger air bag (PAB) which is mounted to an instrument panel in front of a passenger's seat for protecting the passenger in a passenger's seat. Basically, the driver air bag is requisite, and the passenger air bag is optional; however, vehicles with passenger air bags basically mounted therein for protecting passengers from injury are on the increase in recent years. 
FIG. 1 is a cross sectional view of an air bag module according to the conventional art.
As shown in FIG. 1, the air bag module comprises: an air bag housing 2 fixed to a vehicle; a cushion 4 accommodated in the front part of the air bag housing 4; and an inflator mounted to the rear part of the air bag housing 2 for supplying a high pressure expansion gas so that the cushion 4 is expanded toward a passenger when a collision having a severity greater than a predetermined severity occurs.
A retainer 8 having a plurality of retainer holes 8a for passing the expansion gas supplied by the inflator 6 through is disposed between the cushion 4 and the inflator 6.
The expansion pressure of the inflator 6 is adjusted according to the type of a passenger in accordance with a physical condition of the passenger.
The operation of the thus-constructed air bag module according to the conventional art will now be described.
When a collision having a severity greater than a predetermined severity occurs, a high pressure expansion gas is emitted from the inflator 6. The high pressure expansion gas emitted from the inflator 6 fills the cushion 4. As the cushion 4 inflates like a balloon by the high pressure expansion gas supplied from the inflator 6, it is expanded toward a passenger's seat. Then, as illustrated in FIG. 2, the passenger sitting on the passenger seat can be protected by the buffering action of the cushion 4.
The conventional air bag module, however, is designed to be perfectly expanded by conforming the size of the cushion 4 to a state in  which an adult man of standard proportions is seated in an upright position. Thus, in case that the expansion pressure of the inflator 6 is adjusted according to the type of a passenger other than an adult man, the expansion size of the cushion 4 is not appropriate, thereby deteriorating the function.
Japanese Laid-Open Patent No. 2001-199299 discloses an air bag module which is provided with an additional cushion capable of adjusting the size of the air bag, supports the additional cushion by a retainer ring and a supporting device, and has a release device for releasing the supporting. However, its structure is complicated and the cost is increased due to the release device.
Korean Laid-Open Patent No. 2003-0016896 discloses an air bag cushion which is provided with an expansion size adjusting portion, with a seam being gradually torn according to a pressure, by sewing parts of the air bag with a thread such that the size of the air bag can be adjusted. However, there is a problem of having to precisely control the strength or thickness of the thread in order to precisely control the tearing of the thread according to a pressure.
U.S. Pat. No. 6,334,627 discloses a variable-volume airbag for an airbag module in which a tab is attached to the part exposed to the outside at the time of expansion of the airbag and the tab is bound with a cord, or in which a tab connected with an islet is attached to the part exposed to the outside at the time of expansion of the airbag and the islet is bound with a cord or hook. However, the passenger may get a bruise caused by the tap, islet, cord and so on at the time of expansion of  the airbag, and a particular process for attaching the tab to the airbag with a thread or the like is required, which makes the working process complicated.